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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 100px;
            height: 100px;
            background-color: red;
           /* 外边距 */
                /*   上   右   下   左   (顺时针) */
            margin: 10px 20px 30px 40px;

           /* 内边距 */
            padding: 20px;


            display: flex;
            flex-direction: row;
            flex-wrap: wrap;
        }
    </style>
</head>

<body>
    <div>123</div>
    <div>245</div>
    <div>353</div>
    <section>
        <a href="#">back</a>
    </section>

</body>

</html>